Before examining the laboratory and field experiments designed to test the joint action of pesticides on fish, this chapter summarizes a few reviews that have greatly helped in clearing the confusion that exists in this subject. It reviews the toxicity of a second compound subsequent to exposure to a first compound, the toxicity of two or more compounds to which a fish is simultaneously exposed, and the methods of calculation of the joint toxicity. There have been a number of reports describing the effect of pre-exposure on the toxicity of the same or a second pesticide to which the organism is exposed. Konemann, while reviewing the various concepts of joint action in vogue, also noted that many definitions used in this field often lead to confusion. This review attempts to point out the inadequacy of the various proposals to calculate the joint action of pesticide mixtures on fish.